1. Field of the Invention
The invention relates to liquid solid separation in general and to thermal desorption in particular.
2. Prior Art
The need to separate solids from liquids arises in many contexts from mining to shipping to waste treatment. Several specialized separation problems are presented in the context of petroleum exploration. In many petroleum exploration and production operations and particularly in applications involving deeper wells, a hydrocarbon based drilling fluid is employed to perform many functions. This drilling fluid generally serves as a hydraulic fluid to drive down hole tools, such as a mud motor, which will turn the drill bit at the leading end of the drill string. The drilling fluid will also serve to lubricate the drill bit.
As the drill bit turns, it will generate cuttings which must be removed from the well bore for the bore to grow. The pressure on the drilling fluid forcing it down the drill string and through the mud motor will also cause the fluid to rise back up the well in the annular space between the drill string and the well wall. The pressure exerted by the drilling fluid will help support the well wall and keep it from collapsing. The flow of the drilling fluid will also carry the cuttings from the well bottom to the surface. Once at the surface, the cuttings will be extracted from the drilling fluid so that the fluid may be reused in operation of the well. However, the separated cuttings will be soaked in the hydrocarbon based drilling fluid, which by that point, may also include water and/or petroleum emitted by the well. Generally, at this stage the separated cuttings will be contained in a thick oil based slurry.
The condition of the cuttings creates disposal problems. During drilling many, many tons of cuttings will be produced. In offshore operations, environmental regulations as well as principles of sound environmental stewardship prevent operators from simply dumping the cuttings overboard, since to do so would result in the discharge of a substantial quantity of oil into the water. Similarly, in land based operations, the easiest thing to do with the cuttings would be to bury them. However, the presence of the oil based fluids on the cuttings make this impossible because of the risk that the oil will contaminate ground water supplies. Thus, the oil must be separated from the cuttings before the cuttings can be disposed.
Because of the quantities and distances involved, transporting the cuttings to a treatment facility can be prohibitively expensive. Thus, treatment on site is preferable. However, treatment on site creates a whole new set of difficulties.
In many petroleum wells, a substantial amount of natural gas is generated. Sometimes the gas is the primary intended product of the well. Sometimes gas is extracted as a bi-product of a well whose primary product is crude. In either case, gas is often present during exploration and production.
Natural gas is highly flammable. While modem wells are designed to capture the gas released during exploration and production, there is always a risk that the system for capturing the gas will fail—known in the field as a “blow out.” A blow out creates a serious fire and explosion hazard for well operators, and as a result extreme care must be taken to ensure that no potential flash points are present. The care shown by rig operators in this case is a result of both sound rig safety policies and government safety regulations.
Separation of the cuttings and the oil based drilling fluids usually involves heat. For example, one common separator is a rotary kiln, in which electric current is passed through a heating coil. However, such prior art separators often have excessively hot portions that are exposed to the atmosphere. If a gas cloud were released in a blow out, such exposed portions of the separators could potentially be a flash point capable of igniting the gas cloud and causing fires and/or explosions. Such hot spots make the presence of many prior art separators at petroleum production facilities potentially dangerous.
One common industry solution to this risk is to build separate platforms or platform extensions for the separator at a distance from the location of any likely gas discharge. Given the premium for space in offshore exploration and production operations, such solutions can be very expensive. Moreover, depending upon wind and other environmental conditions at the time of a blow out, physical separation of the separator may not be sufficient to entirely eliminate the risk posed unless the separation distance is very great.
In addition to the foregoing shortcomings, such prior art separators are also often relatively costly and inefficient. Therefore, a liquid solid separator meeting the following objectives is desired.